If you don't have a bachelor's degree, enroll in a university and earn a degree in a related field such as English or second-language acquisition. You should also take some education methodology courses. Take courses in a second language so that you can know first-hand what a student of another language has to experience. If you plan to teach as a career, then a four-year degree with the appropriate education courses would be the best course to take.
Obtain a teaching certificate in your state. Apply to your state's Department of Education and become certified in your teaching area. This will enable you to gain some experience in the classroom before you start teaching ESOL. If you enter teaching with a four-year degree without education courses you will need to take some night classes to earn the education requirement, but at the same time you can be gaining valuable teaching experience.
Enroll in an ESOL or TESOL (the T stands for "teaching") certificate program at a university. Most universities look for at least a master's degree when seeking instructors, so you will probably have to get a graduate degree. Course lengths will vary around the world; for instance, in Australian universities a master's in TESOL will take two semesters. In the United States, you can do an intensive ESOL course in just one semester. Numerous colleges offer short, intensive courses, but you need to make sure your school has all the necessary certifications.
Apply to a college or university for a job as an ESOL instructor. You should expect to apply only for part-time work, as most colleges and universities hire full-time staff from within. You may need to work a few semesters as an adjunct teacher before being considered for a full-time post. Par- time posts will generally have two or three classes of about three hours per day. Once you are on the faculty of a college you can apply for permanent full-time work as openings arise.